The present invention relates to a motor, in particular a linear motor for a machine tool or a pump, wherein the movement of the motor can be controlled without requiring correction of the movement of a reaction section.
Workpieces or tools are moved in many machines in industrial production processes, with the movement taking place along fixed, predetermined axis tracks. The rate at which the machine operates depends essentially on the speed of the adjustment and transport movements. High axis speeds are thus desirable in order to achieve high machine productivities.
In most cases, the adjustment movements, which need to have a certain precision, are carried out by controlled electrical servo drives. In order to take account of the considerably more stringent requirements for accuracy and speed, direct drives have been increasingly used for many years for high-precision adjustment and transport movements. In direct drives, the feed or rotation force required for the movement is produced by a motor or a power converter and is introduced into the useful load without passing through any gearbox or mechanical transmissions. Appropriate servo motors and linear motors are available. The high accelerations and major jolts (rate of change of acceleration) which occur with rapid axis movements lead to vibration in the machine structures, and thus to negative influences on the processing results.
Like all motors, direct drives also have two mutually paired active surfaces, between which the drive force is built up as a consequence of the feed to the motor. The motor power or the motor torque is essentially proportional to the magnitude of the feed, or is linearized by suitable electronic distortion compensation. The feed side of the motor is referred to as the xe2x80x9cactive sidexe2x80x9d. As an example, a linear motor will be considered here, whose active section is also called the xe2x80x9cprimary sectionxe2x80x9d. Both arrangements in which the active section is connected to the useful load and arrangements in which the passive section drives the useful load are in widespread use. That motor section which faces the useful load is referred to as the xe2x80x9coutput-drive sidexe2x80x9d of the motor. The side of the motor facing away from the useful load is referred to as the reaction side in the following text.
In order to transport the useful load as precisely as possible to the predetermined set positions, the actual position is detected by means of a position measurement system, and is supplied to a control unit. The position errors are evaluated here, and a calculated motor power is applied in order to accelerate the useful load so that the undesirable position error is reduced again. This process is referred to as control.
DE 23 54 947 discloses a linear motor which is based on vibration-damping bell elements and which is used in areas in which there is no requirement for high movement precision, for example for hoists, crane carriages, etc. This bearing ensures that the motor starts xe2x80x9csmoothlyxe2x80x9d.
However, in machine construction, it is assumed that drive units which are intended to carry out precise and rapid movements need to make contact with their reference masses in as fixed (rigid) a manner as possible. It is therefore always desirable to couple the motor output-drive side to the useful load in as fixed a manner as possible and to couple the motor reaction side to the base in as hard a manner as possible. In order to achieve this, DE 297 18 566, for example, provides for the motor reaction side to be coupled to the base via a material providing a high level of vibration damping (considerably greater than that of steel). It is likewise of critical importance to connect the position sensors, which are part of the control system described above, to the reference bodies in as fixed a manner as possible, and with as little vibration as possible.
During dynamic movement processes, the predetermined route curves produce high momentum changes, which must be transmitted through the direct drive to the useful load. Since the momentum is maintained, the motor reaction side has to absorb the opposing momentum. This momentum is introduced directly into the base, owing to the hard connection. This leads to severe vibration of the machine reference body, for example the machine bed, which may have disadvantageous effects on the accuracies and surface quality of the processing.
The article xe2x80x9cA Fast-Tool-Servo System based on electrodynamic and piezoelectric Actuatorsxe2x80x9d (Annals of the German Academic Society for Production Engineering 2 (1996), no. 2 and CIRP Annals. Manufacturing technology 1 (1995)) proposes that the drive system in a processing machine be softly coupled to the base. However, the relative reference between the output-drive section and the reaction section is measured by the measurement system, by means of a linear tacho. The result of the measurement is then supplied to the control unit. This allows the control system to have direct access to the relative movement between the output-drive section (useful mass or tool) and the base, since the movement of the reaction section is superimposed on the movement of the output-drive section, relative to the fixed base. The movement of the reaction section must therefore be corrected numerically in the position data, for which reason the soft coupling of the drive system to the base must have a behavior which is constant and can be defined in absolute terms. Even very minor discrepancies in the coupling parameters lead to major errors in the useful load movement.
The invention is based on the object of providing a motor of this generic type for machine tools or the like, which is designed such that it can be controlled in a simple manner, particularly without requiring any numerical correction for the movement of the reaction section.
According to one aspect of the invention, the drive motor for a machine tool includes an output-drive section associated with a load, a reaction section which interacts with the output-drive section, a base for attaching the drive motor to the machine tool, a bearing apparatus disposed between the reaction section and the base, with the reaction section mounted on the bearing apparatus such that the reaction section performs a movement that opposes movement of the output-drive section, and a position sensing device for deriving data for position control of the drive motor, wherein the position sensing device determines position data of the output-drive section relative to the base.
The invention is based on the surprising knowledge that precise tool movement control is possible despite the reaction section being decoupled from the base. The consistent use of the characteristics of a direct drive according to the invention, namely power conversion, allows an additional degree of freedom to be derived from this, which allows the predetermined drive momentum to be transmitted to the useful load without the base being loaded by the unavoidable opposing momentum. The present invention makes use of the characteristic that a power converter, such as a synchronous motor with permanent-magnet excitation, converts the feed variablexe2x80x94in this case the currentxe2x80x94into a drive force proportional to this feed variable, directly and without any delay. In the case of power converters, such as the synchronous electric motor which is used as an example here, the drive force is dependent essentially only on the feed variable. It is significant that the present position of the motor active surfaces with respect to one another and, above all, their present speed with respect to one another, have no significant influence on the motor power. In high-quality power converters, the position-dependent and speed-dependent influences on the motor power are negligible.
The power converter thus always provides a drive force which is dependent exclusively on the feed variable used at that time, for example the current, but not on the speed or the position of the motor active surfaces. If a drive axis is set up according to the invention, in which the reaction side of the motor may move, the reaction side moves in the opposite direction, under the influence of the reaction momentum, when a nominal pulse is applied, to the useful load. The speed of the motor reaction side due to the opposing pulse is governed by the mass ratio of the two masses involved in the momentum transmission. The common center of gravity of the power converter system (with the masses coupled to it), in contrast, does not change speed. Owing to said lack of dependency of the motor power on the relative speed of the motor active surfaces, the actual momentum transmitted to the useful load is not influenced in any way by the opposing movement of the motor reaction side. The same position sensors and control strategies can thus be used in exactly the same way as the rigidly linked power converters in the known machines as well. In particular, the reference points of the shaft position sensor which, as is known, are of major importance for the quality of movement control, remain identical. The relative position between the moving useful load and the base, which is in a fixed position with respect to the axis movement direction, is thus measured in the same way.
Owing to the position measurement of the output-drive section with respect to the fixed-position base, the control system for the motor receives only data that do not include the movement of the reaction section. There is thus no need for numerical compensation for the movement of the reaction section with respect to the base. The control process is considerably simplified in this way. Since, furthermore, the reaction section is decoupled from the base and thus no longer transmits any momentum to it, positions can be determined considerably more accurately. Specifically, any momentum transmission to the base would lead to vibration of the base. Such vibration of the base would likewise be reflected in the position data, and would thus increase the fluctuation range of such data. This would in turn make position control more difficult. The motor according to the invention thus allows considerably much more accurate position control to be carried out than is possible with motors according to the prior art.
The only factor that need be remembered is that, in those power converters which use a multiphase feed, such as widely-used three-phase electric motors, the motor phase angle of the feed, must be related in a fixed way to the relative position (and, if necessary, to the relative speed as well) of the motor active surfaces. In the case of the synchronous motors used here by way of example, this process is called xe2x80x9ccommutationxe2x80x9d. In these power converters, the relative position of the motor active surfaces with respect to one another must be tapped off by means of a sensor, and supplied to the motor control system. It should be stressed in particular that the readjustment of the phase angle of the feed described here has nothing to do with the position control system of the power converter, but is used only to maintain the power converter constants.
If the machine is designed such that movement of the motors reaction side is permissible, then the intrinsic mass of this motor reaction side in fact absorbs the reaction, and the machine foundation is relieved from absorbing the reaction. This is particularly advantageous, for example, in the case of drives for adjusting the cutting tool of eccentric lathes, in which the cutting tool oscillates with extremely high accelerations and relatively small amplitudes in order to follow the eccentric contour of the workpiece as it rotates at high speed. In this case, the motor reaction side carries out an opposing oscillation, whose amplitude is likewise relatively small. The machine bed is not affected by shaking momentums, since these will have been dissipated in the opposing oscillation of the motor reaction side.
If the motor reaction side were suspended completely freely in the axis movement direction, its opposing movements could increase without any limitation. In particular, it is impossible to transmit forces which act in the same direction over a relatively long time. The cutting forces of processing processes, for example, could not be transmitted with completely free motor reaction side suspension. However, this problem can be overcome according to one preferred embodiments of the invention, if the motor reaction side is not suspended completely freely but is anchored to the base via an elastic member, for example a spring element. The mean deflection of the motor reaction side is then governed by the force balance between the constant element of the useful load force and the reaction force of the sprung restoring element. The magnitude of the mean deflection of the motor reaction side can thus be preset by the characteristic of the spring element.
When the motor reaction side is anchored by means of an elastic member, it is obvious that kinetic energy can be stored in the form of oscillations by the motor reaction side with respect to the base. A characteristic feature of an oscillating system is its resonant frequency. This is governed by the ratio of the spring temper to the oscillating mass. A particularly interesting fact in this case is that the mass of the driven useful load is not included in the mass that needs to be considered for the resonant frequency since, owing to its characteristics described above, the power converter cannot form any intrinsically stable restoring forces whatsoever between the two motor active surfaces. This behavior is desirable and leads to the useful load being perfectly decoupled from the oscillations of the motor reaction side.
If oscillation frequencies which are close to the natural resonant frequency described above are to be transmitted, it is possible for large oscillation amplitudes, which are typical of resonance phenomena, to occur. This is undesirable. According to a further preferred embodiment, this is corrected by a damper element which is connected in parallel with the spring element. This extracts the energy from the oscillation processes and thus effectively and predictably limits the oscillation amplitude on the motor reaction side. The resonant response can be tuned by means of the damping constant. An aperiodic resonant response has often been found to be particularly advantageous, since this avoids any amplitude peak at resonance.
In this case, the oscillating system behaves as a low-pass filter which, above its cut-off frequency (the resonant frequency), filters the corresponding frequency elements out of the spectrum of the reaction forces of the power converter, and keeps them away from the machine foundation. Magnetic dampers are particularly advantageous as damping elements, since they are free of static and sliding friction, since they operate without any contact.
Depending on their design, modern machines are known to be highly stable in response to disturbance frequencies up to 80 Hz, and excitation frequencies above this should be avoided. This must be remembered when choosing the cut-off frequency of the mechanical low-pass filter. If one remembers that the power amplitude is halved when the frequency is doubled in the damping region of the aperiodically tuned mechanical filter, then it is evident that changing the tuning of the cut-off frequency by values of 5 to 10 Hz is sufficient to achieve highly effective momentum decoupling in the sensitive range above said frequency of 80 Hz. Changing the tuning of the cut-off frequency is, of course, a compromise between the oscillation amplitudes on the motor reaction side, which one would like to permit, and the decoupling level with regard to the reaction impulses in the upper frequency band.
The momentum decoupling method, which will be referred to as a xe2x80x9cStoiber inertia drivexe2x80x9d, offers the machine design an additional degree of freedom, which allows the reaction momentums in direct drives to be effectively kept away from the machine structure, particularly at the higher frequencies, where they reduce accuracy.
In this case, the principle according to the invention is not limited to electric linear motors, but can also be applied to torque motors, fluid motors, etc. It is suitable for all machines in which oscillatory movements need to be provided, for example eccentric lathes and for milling machines, automatic drilling, bonding or component-placement machines, balancing machines, pumps or even loudspeakers.